Solid electrolyte battery

ABSTRACT

A solid electrolyte battery having improved energy density and safety, the solid electrolyte battery incorporating a positive electrode; a negative electrode disposed opposite to the positive electrode; a separator disposed between the positive electrode and the negative electrode; and solid electrolytes each of which is disposed between the positive electrode and the separator and between the separator and the negative electrode, wherein the separator is constituted by a polyolefin porous film, the polyolefin porous film has a thickness satisfying a range not greater than 5 μm nor greater than 15 μm and a volume porosity satisfying a range not less than 25% nor greater than 60%, and the impedance in the solid electrolyte battery is greater than the impedance realized at the room temperature when the temperature of the solid electrolyte battery satisfies a range not less than 100° C. nor greater than 160° C.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a divisional of and claims priority to U.S.application Ser. No. 09/575,237, filed May 22, 2000, now U.S. Pat. No.7,183,021,and also claims priority to Japanese Application No.P11-146653,filed May 26, 1999, and Japanese Application No. P11-365064,filed Dec. 22, 1999, which applications are incorporated herein byreference to the extent permitted by law.

BACKGROUND OF THE INVENTION

The present invention relates to a solid electrolyte batteryincorporating a solid electrolyte, and more particularly to a solidelectrolyte battery incorporating a separator which has specificmechanical strength and thermal characteristics so as to considerablyimprove its energy density and safety.

As a power source for a portable electronic apparatus, such as aportable telephone or a notebook personal computer, a battery is animportant element. To reduce the size and weight of the electronicapparatus, increase in the capacity of the battery and reduction in thevolume of the same have been required. From the foregoing viewpoints, alithium battery exhibiting a high energy density and output density issuitable to serve as the power source of the portable electronicapparatus. A lithium battery incorporating a negative electrode made ofa carbon material has a mean discharge voltage of 3.7 V or greater.Moreover, deterioration caused from charge and discharge cycles canrelatively satisfactorily be prevented. Therefore, the lithium batteryhas an advantage that a high energy density can easily be realized.

Lithium batteries are required to permit a variety of shapes to beformed. The batteries have the flexibility and high degree of freedom oftheir shape to form a sheet battery having a small thickness and a largearea and a card battery having a small thickness and a small area. Aconventional structure, in which battery elements—a positive electrode,a negative electrode, and an electrolytic solution—are enclosed in ametal can, encounters difficulties in forming the variety of shapes.Since the electrolytic solution is employed, the manufacturing processbecomes too complicated. Moreover, a countermeasure against leakage ofthe solution must be taken.

To solve the above-mentioned problems, batteries have been researchedand developed which incorporate a solid electrolyte composed of either aconductive organic polymer or inorganic ceramic solid electrolyte or agel-like solid electrolyte (hereinafter called a “gel electrolyte”) inwhich matrix polymers are impregnated with electrolytic solution. Inboth types of solid electrolyte batteries the electrolyte is fixed.Therefore, contact between the electrode and the electrolyte can bemaintained. Hence it follows that the foregoing batteries are free fromthe necessity of having to enclose the electrolytic solution byemploying a metal can or by exerting pressure on the battery element. Afilm-shape case material can be used to reduce the thickness of thebattery. Thus, an energy density greater than that of a conventionalbattery can be realized.

In general, the solid electrolyte of the solid electrolyte battery hasproper mechanical strength as disclosed in “MATERIAL TECHNIQUE OFHIGH-PERFORMANCE SECONDARY BATTERY AND EVALUATION, APPLICATION ANDDEVELOPMENT OF THE SAME” (Technical Information Association, 1998).Therefore, a structure of the battery, distinct from that of theconventional battery incorporating the electrolytic solution, can beselected. For example, it has been reported that a separator is notrequired between the positive electrode and the negative electrode. Thisprovides a known advantage for the solid electrolyte.

The reported solid electrolyte suffers from unsatisfactory strength,including piercing resistance, as compared with the conventionalseparator constituted by a polyolefin porous film and the like. When thethickness of the solid electrolyte of the conventional solid electrolytebattery is reduced to, for example, 40 μm or greater to raise the energydensity, there arises a problem in that internal short circuitingfrequently occurs after the battery has been assembled. As describedabove, the energy density of the solid electrolyte battery cannot easilybe raised by reducing the thickness of the solid electrolyte layer.

As for heat resistance, which is an index to evaluate the reliability ofthe battery, the conventional solid electrolyte battery suffers fromunsatisfactory heat resistance. A portion of the batteries on the marketare designed to use a so-called “shutdown effect” to improve heatresistance. However, a solid electrolyte material for the solidelectrolyte battery having the shutdown effect has not been found.

As for the reliability and safety of the battery, the reliability andsafety cannot easily be realized as the energy density of the battery israised. Therefore, a technique for maintaining the safety of the solidelectrolyte battery also must be considered when a solid electrolytebattery is designed to raise the energy density.

A thin battery incorporates a separator which is made of polyolefin. Inparticular, a polyethylene separator is employed.

In a usual state, when the temperature of the battery has melted downand, therefore, short circuiting occurs between the positive electrodeand the negative electrode, thermorunaway does not occur. In a casewhere a battery is used in an abnormal environment, for example, in acase where the temperature of a battery has been raised because thebattery has been charged to a voltage level greater than a usual level,there is apprehension that an accident can occur. In the foregoing case,there is apprehension that use of a separator made of polyethylene,which has a melting point less than that of polypropylene, might cause amelt-down of the separator to take place. That is, breakage of theseparator might occur, causing a short circuit between the positiveelectrode and the negative electrode to take place. Thus, there isapprehension that the battery will generate heat.

BRIEF SUMMARY OF THE INVENTION

In view of the foregoing, an object of the present invention is toprovide a solid electrolyte battery having a high energy density andimproved safety.

To achieve the object, according to one aspect of the invention, thereis provided a solid electrolyte battery including: a positive electrode;a negative electrode disposed opposite to the positive electrode; aseparator disposed between the positive electrode and the negativeelectrode; and solid electrolytes each of which is disposed between thepositive electrode and the separator and between the separator and thenegative electrode, wherein the separator is constituted by a polyolefinporous film, the polyolefin porous film having a thickness of not lessthan 5 μm nor greater than 15 μm and a volume volume porosity not lessthan 25% nor greater than 60%, and the impedance in the solidelectrolyte battery is greater at a temperature not less than 100° C.nor greater than 160° C. than the impedance at room temperature.

The solid electrolyte battery according to the present inventionincorporates the separator constituted by the polyolefin porous filmhaving the specified thickness, volume volume porosity and thermalcharacteristic. Thus, the energy density can be raised and the safety ofthe same can be improved.

According to another aspect of the present invention, there is provideda solid electrolyte battery including: a positive electrode; a negativeelectrode disposed opposite to the positive electrode; a separatordisposed between the positive electrode and the negative electrode; andsolid electrolytes each of which is disposed between the positiveelectrode and the separator and between the separator and the negativeelectrode, wherein the separator is constituted by a polyolefin porousfilm, the polyolefin porous film has thickness not less than 5 μm norgreater than 15 μm, a volume volume porosity not less than 25% norgreater than 60%, a breaking strength less than 1650 kg/cm² and abreaking ductility not less than 135%.

The solid electrolyte battery according to the present inventionincorporates the separator constituted by the polyolefin porous filmhaving the specified thickness, volume volume porosity and thermalcharacteristic. Thus, the energy density can be raised and the safety ofthe same can be improved.

According to another aspect of the present invention, there is provideda solid electrolyte battery including: a positive electrode; a negativeelectrode disposed opposite to the positive electrode; a separatordisposed between the positive electrode and the negative electrode; andsolid electrolytes each of which is disposed between the positiveelectrode and the separator and between the separator and the negativeelectrode, wherein the separator is constituted by a composite materialof polyethylene and polypropylene, the polyolefin porous film has athickness not less than 5 μm nor greater than 15 μm, the shutdowntemperature is substantially the same as the shutdown temperature of aseparator constituted by polyethylene, and the meltdown temperature isgreater than the meltdown temperature of a separator constituted bypolypropylene by not less than 10° C. nor greater than 30° C.

The solid electrolyte battery according to the present inventionincorporates the separator constituted by a composite material ofpolyethylene and polypropylene. Thus, the energy density can be raisedand the safety of the same can be improved.

According to another aspect of the present invention, there is provideda solid electrolyte battery including: a positive electrode; a negativeelectrode disposed opposite to the positive electrode; a separatordisposed between the positive electrode and the negative electrode; andsolid electrolytes each of which is disposed between the positiveelectrode and the separator and between the separator and the negativeelectrode, wherein the separator is formed by bonding a first separatorconstituted by polyethylene and a second separator constituted bypolypropylene to each other, the separator has a thickness not less than5 μm nor greater than 15 μm, and the separator has a shutdowntemperature which is substantially the same as the shutdown temperatureof a separator constituted by polyethylene, and a meltdown temperaturewhich is substantially the same as the meltdown temperature of aseparator constituted by polypropylene.

The solid electrolyte battery according to the present inventionincorporates the separator formed by bonding the first separatorconstituted by polyethylene and a second separator constituted bypolypropylene to each other. Thus, the energy density can be raised andthe safety of the same can be improved.

Other objects, features and advantages of the invention will be evidentfrom the following detailed description of the preferred embodimentsdescribed in conjunction with the attached drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a perspective view showing an example of the structure of asolid electrolyte battery according to the present invention;

FIG. 2 is a cross sectional view taken along line X-Y shown in FIG. 1;

FIG. 3 is a perspective view showing a state where a positive electrodeand a negative electrode are formed into a wound electrode;

FIG. 4 is a perspective view showing an example of the structure of thepositive electrode;

FIG. 5 is a perspective view showing an example of the structure of thenegative electrode;

FIG. 6 is a photograph showing a fibril structure of a separatoraccording to the present invention;

FIG. 7 is a perspective view showing an example of the structure of thesolid electrolyte battery according to the present invention;

FIG. 8 is a cross sectional view taken along line X-Y shown in FIG. 7;

FIG. 9 is a perspective view showing a state where a positive electrodeand a negative electrode are formed into a wound electrode;

FIG. 10 is a perspective view showing an example of the structure of thepositive electrode;

FIG. 11 is a perspective view showing an example of the structure of thenegative electrode;

FIG. 12 is a cross sectional view showing an example of a separatoraccording to the present invention;

FIG. 13 is a schematic view showing the relationship between the widthof the separator and that of the electrode;

FIG. 14 is a cross sectional view showing an example of the structure ofa casing film;

FIG. 15 is a graph showing the relationship between the temperature ofthe battery according to Example 1 and the internal impedance of thesame;

FIG. 16 is a graph showing the relationship between the temperature ofthe battery according to Example 6 and the internal impedance of thesame;

FIG. 17 is a graph showing the relationship between the breakingstrength and the breaking ductility of each of the separators of thebatteries according to Examples 1 to 7; and

FIG. 18 is a photograph showing the fibril structure of the separator ofthe battery according to Example 6.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will now be described.

An example of the structure of a gel electrolyte battery 1 according tothis embodiment is shown in FIGS. 1 and 2. The gel electrolyte battery 1incorporates an elongated positive electrode 2; an elongated negativeelectrode 3 disposed opposite to the positive electrode 2; a gelelectrolyte layer 4 formed on each of the positive electrode 2 and thenegative electrode 3; and a separator 5 disposed between the positiveelectrode 2 having the gel electrolyte layer 4 formed thereon and thenegative electrode 3 having the gel electrolyte layer 4 formed thereon.

The gel electrolyte battery 1 has a structure such that the positiveelectrode 2, having the gel electrolyte layer 4 formed thereon, and thenegative electrode 3, having the gel electrolyte layer 4 formed thereon,are laminated through the separator 5. Moreover, a wound electrode 6shown in FIG. 3 is formed by winding the positive electrode 2 and thenegative electrode 3 in a lengthwise direction. The wound electrode 6 iscovered with a casing film 7 so as to be hermetically sealed. Apositive-electrode terminal 8 is connected to the positive electrode 2,while a negative-electrode terminal 9 is connected to the negativeelectrode 3. The positive-electrode terminal 8 and thenegative-electrode terminal 9 are sandwiched in a sealing region whichis the outer periphery of the casing film 7. Each of the regions inwhich the positive-electrode terminal 8 and the negative-electrodeterminal 9 are in contact with the casing film 7 is provided with aresin film 10.

As shown in FIG. 4, the positive electrode 2 incorporates apositive-electrode active material layer 2 a containing apositive-electrode active material and formed on each of the two sidesof a positive-electrode collector 2 b. The positive-electrode collector2 b is constituted by, for example, a metal foil, such as an aluminumfoil.

The positive-electrode active material may be a composite lithium oxide,such as cobalt acid lithium, nickel acid lithium or spinel manganeseacid lithium. The composite lithium oxide may be employed solely or aplurality of the foregoing materials may be employed.

It is preferable that the composite lithium oxide has a mean particlesize of 15 μm or less. When the composite lithium oxide having the meanparticle size of 15 μm or less is employed as the positive-electrodeactive material, a gel electrolyte battery can be obtained which has lowinternal resistance and excellent output characteristics.

FIG. 4 shows a state where a gel electrolyte layer 4 to be describedlater has been formed on the positive-electrode active material layer 2a of the positive electrode 2.

As shown in FIG. 5, the negative electrode 3 incorporate anegative-electrode active material layer 3 a containing anegative-electrode active material and formed on each of two sides of anegative-electrode collector 3 b. The negative-electrode collector 3 bis constituted by a metal foil, such as a copper foil.

The negative-electrode active material may be a material to whichlithium can be doped/dedoped. The material to which lithium can bedoped/dedoped may be lithium, its alloy or a carbon material.Specifically, the carbon material is exemplified by carbon black, suchas natural graphite, artificial graphite, pyrocarbon, cokes or acetyleneblack; vitreous carbon; active carbon; carbon fiber; a sintered materialof an organic polymer; a sintered material of coffee beans; a sinteredmaterial of cellulose; or a sintered material of bamboo.

The inventor has energetically performed studies. As a result, a facthas been detected that methocarbon microbead carbon graphitized at abaking temperature of about 2800° C. is a preferred material. Themethocarbon microbead carbon has high electro-chemical stability withrespect to an electrolytic solution. Therefore, an effect can beobtained when it is combined with a gel electrolyte of a type adapted toan electrolytic solution containing propylene carbonate.

It is preferable that the methocarbon microbead carbon has a meanparticle size from 6 μm to 25 μm. As the mean particle size of themethocarbon microbead carbon is reduced, the overvoltage in theelectrode reaction can be reduced. As a result, the outputcharacteristics of the battery can be improved. To raise the electrodefilling density, it is advantage to enlarge the mean particle size.Therefore, it is preferable that methocarbon microbead carbon having amean particle size from 6 μm to 25 μm is employed.

FIG. 5 shows a state where the gel electrolyte layer 4 to be describedlater has been formed on the negative-electrode active material layer 3a of the negative electrode 3.

The gel electrolyte layer 4 contains an electrolyte salt, a matrixpolymer and a swelling solvent which serves as a plasticizer.

The electrolyte salt may be any one of LiPF₆, LiClO₄, LiCF₃SO₃, LiAsF₆,LiBF₄, LiN(CF₃SO₃)₂ and C₄F₉SO₃Li which may be employed solely or theircombination may be employed. In particular, it is preferable that LiPF₆is employed from a viewpoint of obtaining satisfactory ion conductivity.

The matrix polymer must have an ion conductivity of 1 mS/cm or greaterat room temperature whether used as a sole polymer or used in the formof a gel electrolyte. When the foregoing ion conductivity is realized,the chemical structure of the matrix polymer is not limited. The matrixpolymer is exemplified by polyvinylidene fluoride, polyacrylonitrile,polyethylene oxide, a polysiloxane compound, a polyphosphagen compound,polypropylene oxide, polymethylmethacrylate, polymethacrylonitrile and apolyether compound. Also a material obtained by copolymerizing anotherpolymer with the foregoing polymers may be employed. From a viewpoint ofrealizing chemical stability and ion conductivity, a material isemployed constituted of polyvinylidene fluoride and hexafluoroproylene,such that hexafluoropropylene is contained in a quantity less than 8%.

The swelling solvent may be a nonaqueous solvent exemplified by ethylenecarbonate, propylene carbonate, γ-butylolactone, acetonitrile,diethylether, diethyl carbonate, dimethyl carbonate,1,2-dimethoxyethane, dimethylsulfoxide, 1,3-dioxolane, methylsulfomate,2-methyltetrahydrofuran, tetrahydrofuran, sulfolane, 2,4-difluoroanisoland vinylene carbonate. The foregoing materials may be employed solelyor their mixture may be employed.

In particular, it is preferable that a material, such as ethylenecarbonate, propylene carbonate or γ-butylolactone, having a relativelywide potential window, is employed. Note that the potential window is apotential region in which the solvent is stably present.

When 2,4-difluoroanisol or vinylene carbonate is added in a quantityfrom 0.5% to 5% of the overall weight of the solvent, thecharacteristics of the battery sometimes can be improved.

It is preferable that the gel electrolyte layer 4 has a structure suchthat the ratio of the matrix polymer to the swelling solvent is not lessthan 1:5 nor greater than 1:10. When the quantity of the swellingsolvent is greater than five times the quantity of the matrix polymer,the electrolytic solution component in the gel electrolyte is too small.Thus, the ion conductivity of the gel electrolyte layer 4 deteriorates.When the quantity of the swelling solvent is greater than 10 times thequantity of the matrix polymer, the gel electrolyte becomes brittle.Thus, satisfactory liquid holding performance of the matrix polymercannot be obtained.

When the ratio of the matrix polymer to the swelling solvent satisfiesthe foregoing range, the liquid holding performance of the matrixpolymer can be maintained. Moreover, the ion conductivity of the gelelectrolyte layer 4 can be maintained.

It is preferable that the thickness of the gel electrolyte layer 4 isnot less than 5 μm nor greater than 19 μm. When the thickness of the gelelectrolyte layer 4 is less than 5 μm, a quantity of gel required tocause the electrode reaction to proceed smoothly cannot easily beobtained. When the thickness of the gel electrolyte layer 4 is greaterthan 19 μm, the distance between the positive electrode 2 and thenegative electrode 3 is elongated. As the distance between the twoelectrodes is elongated, the energy density of the battery and theoutput characteristics of the same deteriorate excessively. Therefore,the thickness of the gel electrolyte layer 4 is made to be not less than5 μm nor greater than 19 μm. Thus, deterioration in the energy densityof the battery and that of the output characteristics can be preventedduring the reaction of the electrode.

The separator 5 disposed between the positive electrode 2 and thenegative electrode 3 prevents short circuiting caused from physicalcontact between the positive electrode 2 and the negative electrode 3.

The thickness of the separator 5 according to this embodiment is notless than 5 μm nor greater than 15 μm. When the thickness of theseparator 5 is less than 5 μm, the separator 5 cannot easily be handledduring a process for manufacturing the battery. As a result, themanufacturing yield of the gel electrolyte battery 1 deteriorates. Whenthe thickness of the separator 5 is greater than 15 μm, the internalresistance of the gel electrolyte battery 1 is raised excessively. Whatis worse, the energy density loss is increased. Therefore, the thicknessof the separator 5 is not less than 5 μm nor greater than 15 μm. Henceit follows that deterioration in the manufacturing yield of the gelelectrolyte battery 1, increase in the internal resistance, and increasein the energy density loss all can be prevented.

The volume porosity of the separator 5 according to this embodiment isnot less than 25% nor greater than 60%. When the volume porosity of theseparator 5 is less than 25%, the internal resistance of the gelelectrolyte battery 1 is so great that it prevents obtaining therequired output characteristics. When the volume porosity of theseparator 5 is greater than 60%, satisfactory mechanical strength cannoteasily be realized. Therefore, the volume porosity of the separator 5 ismade to satisfy a range not less than 25% nor greater than 60%. Thus,the mechanical strength of the separator 5 can be maintained without anyrise in the internal resistance of the gel electrolyte battery 1.

The separator 5 according to this embodiment has a shutdown effect whenthe temperature of the battery is not less than 100° C. nor greater than160° C. To obtain the shutdown effect when the temperature of thebattery is not less than 100° C. nor greater than 160° C., the meltingpoints of the materials constituting the separator 5 must be not lessthan 100° C. nor greater than 160° C. Since the separator 5 is disposedbetween electrodes, the separator 5 must have electrochemical stability.

“A shutdown effect obtained when the temperature of the separator 5 isnot less than 100° C. nor greater than 160° C.” means that the internalimpedance of the battery is increased, as compared with the internalimpedance at room temperature, by 10° C. or more when the temperature ofthe battery is not less than 100° C. nor greater than 160° C.

As a material which satisfies the foregoing conditions, a polyolefinpolymer is a representative material which is exemplified bypolyethylene or polypropylene. In particular, it is preferable that theseparator 5 is made of polyethylene. As an alternative to the polyolefinpolymer, resin of a type having chemical stability with respect to thegel electrolyte may be employed such that the resin is copolymerized orblended with polyethylene or polypropylene.

As described above, the separator 5 has a thickness not less than 5 μmnor greater than 15 μm, a volume porosity not less than 25% nor greaterthan 60%, and a shutdown effect when the temperature is not less than100° C. nor greater than 160° C. Thus, the energy density of the gelelectrolyte battery 1 can be raised and safety can be improved.

Moreover, the inventor of the present invention has energeticallystudied the relationship between the physical properties of theseparator 5 and the characteristics of the battery. As a result, thefollowing has been detected: it is preferable that the separator 5 havea thickness not less than 5 μm nor greater than 15 μm, a volume porositynot less than 25% nor greater than 60%, a breaking strength less than1650 kg/cm², and a breaking ductility of 135% or more. When the breakingstrength and the breaking ductility of the separator 5 cannot satisfythe foregoing ranges, the separator 5 cannot easily be handled in theprocess for manufacturing the gel electrolyte battery 1. Thus, themanufacturing yield of the gel electrolyte battery 1 deteriorates. Whatis worse, satisfactory characteristics of the battery cannot beobtained. Therefore, employment of the separator 5 having the breakingstrength less than 1650 kg/cm² and the breaking ductility of 135% ormore enables deterioration in the manufacturing yield of the gelelectrolyte battery 1 to be prevented. Thus, satisfactorycharacteristics of the gel electrolyte battery 1 can be obtained.

A tensile test of the separator 5 to evaluate the breaking strength andthe breaking ductility of the separator 5 will now be described.

A test piece of the separator 5 substantially in the form of a 30mm×70mm rectangle is obtained by cutting. Then, a cellophane tape having awidth of 10 mm is applied to each of the two lengthwise ends of the testpiece.

Then, the obtained test piece is sufficiently clamped in a sampleclamping portion of a tensile testing machine. The tensile testingmachine may be, for example, model NO. 1310f manufactured by Aiko. Theportion of the test piece to be clamped by the sample clamping portionis the portion which is reinforced with the cellophane tape. That is,the portion of the test piece having a length of 50 mm, except for thetwo-end reinforced portions each having the length of 10 mm, issubjected to the tensile test.

In the foregoing state, that the test piece is placed perpendicularly tothe testing machine table is confirmed. Then, the tensile test isstarted. The tensile strength is 40 mm per minute. Data about the loadand the ductility ratio are recorded in a personal computer through anA/D conversion board. In accordance with obtained data, the breakingstrength and the breaking ductility are calculated. The load with whichthe test piece has been broken is employed as the breaking strength. Thelength (mm) of the test piece immediately before the test piece isbroken is measured to obtain the breaking ductility (%) by usingEquation (1).Breaking Ductility=100×(Length of Test Piece Immediately BeforeBreakage/50)  (1)

The above-mentioned means is performed so that a separator 5 accordingto this embodiment having a breaking strength which is less than 1650kg/cm² and the breaking ductility of 135% or greater can be used. Whenthe separator 5 satisfying the foregoing mechanical characteristics isemployed, deterioration in the manufacturing yield of the gelelectrolyte battery 1 can be prevented. Moreover, satisfactorycharacteristics of the battery can be obtained.

As described above, the separator 5 is employed which has a thicknessnot less than 5 μm nor greater than 15 μm, a volume porosity not lessthan 25% nor greater than 60%, a breaking strength less than 1650kg/cm², and a breaking ductility of 135% or more. Thus, high energydensity and safety of the gel electrolyte battery 1 can be realized.

One of objects of the separator 5 is to prevent short circuiting causedby physical contact between the positive electrode 2 and the negativeelectrode 3. The size of the separator 5 is determined based on thesizes of the positive electrode 2 and the negative electrode 3 and theshapes of the battery elements. That is, the opposite electrodes must becompletely insulated from each other by the separator 5. Moreover, theterminal of the electrode and the electrode must be insulated from eachother by the separator 5. To realize the foregoing state, the size ofthe separator 5 must be greater than the overall size of each of thepositive electrode 2 and the negative electrode 3.

That the separator 5 according to this embodiment has a fine structure,which is a so-called fibril structure as shown in FIG. 6, has beendetected from experiments. FIG. 6 is a photograph of the fine structureof the wound electrode 6 taken by an electronic microscope at amagnification of 50,000 times.

As a means for obtaining the separator 5 having the fibril structure, aplurality of methods may be employed. An example of the method will nowbe described.

Initially, a molten liquid of a low-volatility solvent (a good solventfor the polyolefin composition) is supplied to an extruder containing amolten polyolefin composition, so as to be kneaded. Thus, ahigh-concentration solution of the polyolefin composition having auniform concentration is prepared.

The polyolefin is exemplified by polyethylene and polypropylene. It ispreferable that polyethylene is employed. The low-volatility solvent maybe a low-volatility aliphatic hydrocarbon or a cyclic hydrocarbon, suchas nonane, decane, decalin, p-xylene, undecane or a liquid paraffin.

It is preferable that the weight percentage of the polyolefincomposition is not less than 10 parts by weight nor greater than 80parts by weight, with respect to 100 parts by weight which is the totalquantity of the two materials, preferably not less than 15 parts byweight nor greater than 70 parts by weight. When the quantity of thepolyolefin composition is less than 10 parts by weight, swelling orneck-in occurs excessively at the outlet opening of the die. In theforegoing case, a required sheet cannot easily be formed. When thequantity of the polyolefin composition is greater than 80 parts byweight, a uniform solution cannot easily be prepared. Therefore, theratio of polyolefin is made to satisfy the range not less than 10 partsby weight nor greater than 80 parts by weight. Thus, the preparation ofa uniform solvent and the formation of the sheet can easily beperformed.

A heated solution of the polyolefin composition is extruded through thedie, so that a sheet of the polyolefin composition solution is obtained,which is then cooled. Thus, a gel sheet is obtained. The cooling processis performed until a temperature not higher than the gelling temperatureis realized. As a cooling method, the following methods may be employed:direct contact with cold air, cooling water or other cooling medium; orcontact with a roll cooled with a refrigerant.

The polyolefin composition solution extruded from the die may be takenup at a take-up ratio not less than 1 nor greater than 10, preferablynot less than 1 nor greater than 5. When the take-up ratio is 10 orgreater, great neck-in takes place and breakage easily occurs. When thetake-up ratio is not less than 1 nor greater than 10, neck-in andbreakage of the gel sheet can be prevented.

Then, the obtained gel sheet is heated so as to be oriented at apredetermined stretch ratio, to obtain an oriented film. The gel sheetis oriented by a usual tenter method, a roll method, a milling method ora method of combination of the foregoing methods. It is preferable thata biaxial orientation method is employed. The biaxial orientation methodmay be either of simultaneous orientation in the lengthwise andbreadthwise directions or a sequential method. In particular, it ispreferable that the simultaneous biaxial orientation is employed.

It is preferable that the gel sheet is oriented at a temperature of themelting point of the polyolefin composition+10° C. or less. Morepreferably, the orienting temperature is not less than the crystaldispersion temperature of the polyolefin composition and less than themelting point of the same. When the orienting temperature is greaterthan the melting point of the polyolefin composition+10° C., the resinis undesirably melted. In the foregoing case, effective orientation ofmolecules cannot be realized. When the orienting temperature is lessthan the crystal dispersing temperature, the resin cannot sufficientlybe softened. In the foregoing case, breakage easily occurs in theorienting process and, therefore, orientation at a high stretch ratiocannot be performed. When the orienting temperature of the gel sheet ismade to satisfy the foregoing range, uniform and high-stretch ratioorientation can be performed. Moreover, orientation of the moleculechains can effectively be performed.

The obtained oriented film is cleaned with a volatile solvent to removeresidual low-volatility solvent. The volatile solvent for use in thecleaning process may be a hydrocarbon, such as pentane, hexane orheptane; a hydrogen fluoride, such as ethane trifluoride; or an ether,such as diethylether or dioxane. The foregoing materials are employedsolely or the foregoing materials may be mixed. The solvent for cleaningthe oriented film is selected to be compatible with the low-volatilitysolvent used to dissolve the polyolefin composition.

The oriented film may be cleaned by a method in which the oriented filmis immersed in the solvent to extract the low-volatility solvent left inthe oriented film; a method in which the oriented film is showered withthe solvent; or their combination. The oriented film is cleaned untilthe quantity of the low-volatility solvent left in the oriented film isless than one part by weight.

Finally, the solvent used to clean the oriented film is dried so as tobe removed. The solvent is dried by heating or air spraying. After theforegoing process has been completed, the separator 5 according to thisembodiment can be obtained. The thus-obtained separator 5 has the fibrilstructure as shown in FIG. 6.

The gel electrolyte battery 1 according to this embodiment andincorporating the foregoing separator 5 is prepared as follows.

The positive electrode 2 is prepared as follows: a positive-electrodemix containing the positive-electrode active material and a binder isuniformly applied to the surface of a metal foil, such as an aluminumfoil, which is formed into the positive-electrode collector 2 b, andthen the positive-electrode mix is dried. Thus, the positive-electrodeactive material layer 2 a is formed so that a positive electrode sheetis formed. The binder of the positive-electrode mix may be aconventional binder. Note that conventional additives may be added tothe positive-electrode mix.

Then, the gel electrolyte layer 4 is formed on the positive-electrodeactive material layer 2 a of the positive electrode sheet. To form thegel electrolyte layer 4, a first step is performed so that theelectrolyte salt is dissolved in the nonaqueous solvent. Thus, anonaqueous electrolytic solution is prepared. The matrix polymer isadded to the nonaqueous electrolytic solution, and then the solution issufficiently stirred to dissolve the matrix polymer. As a result, a solelectrolyte solution is prepared.

Then, the electrolyte solution, in a predetermined quantity, is appliedto the surface of the positive-electrode active material layer 2 a.Then, the positive-electrode active material layer 2 a is cooled at roomtemperature so as to gel the matrix polymer. Hence it follows that thegel electrolyte layer 4 is formed on the positive-electrode activematerial layer 2 a.

Then, the positive electrode sheet having the gel electrolyte layer 4formed thereon is cut to obtain elongated members. An aluminum lead wireis welded to a portion of the positive-electrode collector 2 b in whichthe positive-electrode active material layer 2 a is not formed so thatthe positive-electrode terminal 8 is formed. Thus, the elongatedpositive electrode 2 having the gel electrolyte layer 4 formed thereoncan be obtained.

The negative electrode 3 is formed as follows: a negative-electrode mixcontaining a negative-electrode active material and a binder isuniformly applied to a metal foil, such as a copper foil, which isformed into the negative-electrode collector 3 b. Then, the metal foilis dried so that a negative electrode sheet having thenegative-electrode active material layer 3 a formed thereon is prepared.The binder may be a conventional binder. Note that conventionaladditives may be added to the negative-electrode mix.

Then, the gel electrolyte layer 4 is formed on the negative-electrodecollector 3 b. To form the gel electrolyte layer 4, a process similar tothe foregoing process is performed so that an electrolyte solutionprepared similarly to the foregoing process is applied to thenegative-electrode active material layer in a predetermined quantity.Then, the negative-electrode active material layer is dried at roomtemperature to gel the matrix polymer. As a result, the gel electrolytelayer 4 is formed on the negative-electrode collector 3 b.

Then, the negative electrode sheet having the gel electrolyte layer 4formed thereon is cut to obtain elongated members. A lead wireconstituted by, for example, nickel, is welded to a portion of thenegative-electrode collector 3 b in which the negative-electrode activematerial layer 3 a is not formed. Thus, the negative-electrode terminal9 is formed. Thus, the elongated negative electrode 3 having the gelelectrolyte layer 4 formed thereon can be obtained.

The surfaces of the thus-prepared elongated positive electrode 2 and thenegative electrode 3, each having the gel electrolyte layer 4 formedthereon, are disposed opposite to each other. The separator 5 isinserted between the positive electrode 2 and the negative electrode 3to bond and press the laminate. Thus, an electrode laminate is obtained.The electrode laminate is wound in the lengthwise direction so that thewound electrode 6 is obtained.

Finally, the wound electrode 6 is sandwiched in the casing film 7. Then,the resin film 10 is disposed in each region where thepositive-electrode terminal 8, the negative-electrode terminal 9, andthe casing film 7 overlap. Then, the outer periphery of the casing film7 is sealed. Then, the positive-electrode terminal 8 and thenegative-electrode terminal 9 are sandwiched in the sealing opening ofthe casing film 7. Moreover, the wound electrode 6 is hermeticallyenclosed in the casing film 7. After the wound electrode 6 is packed inthe casing film 7, the wound electrode 6 is subjected to heat treatment.Thus, the gel electrolyte battery 1 can be prepared.

When the wound electrode 6 is packed in the casing film 7, the resinfilm 10 is disposed in each of the contact regions between the casingfilm 7 and the positive-electrode terminal 8 and between the casing film7 and the negative-electrode terminal 9. Therefore, the occurrence ofshort circuiting caused by burrs of the casing film 7 or the like can beprevented. Moreover, the adhesion between the casing film 7 and thepositive-electrode terminal 8 and between the casing film 7 and thenegative-electrode terminal 9 can be improved.

The resin film 10 may be constituted by a material adherent to thepositive-electrode terminal 8 and the negative-electrode terminal 9.When the material has the foregoing adhesion, the material is notlimited. It is preferable that any one of polyethylene, polypropylene,denatured polyethylene, denatured polypropylene, their copolymers andpolyolefin resin is employed. It is preferable that the thickness of theresin film 10 realized before thermal bonding is from 20 μm to 300 μm.When the thickness of the resin film 10 is less than 20 μm, handlingdeteriorates. When the thickness is greater than 300 μm, water easilypenetrates the resin film 10. As a result, the airtightness in thebattery cannot be easily maintained.

In the foregoing embodiment, the elongated positive electrode 2 and theelongated negative electrode 3 are laminated. Then, the laminate iswound in the lengthwise direction to form the wound electrode 6. Thepresent invention is not limited to the foregoing structure. The presentinvention may be applied to a structure in which a rectangular positiveelectrode 2 and a rectangular negative electrode 3 are laminated to formthe electrode laminate or a structure in which the electrode laminate isalternately folded.

In the foregoing embodiment, the electrolyte to be interposed betweenthe positive electrode 2 and the negative electrode 3 is the gelelectrolyte containing swelling solvent. The present invention is notlimited to the foregoing structure. The present invention may be appliedto a structure in which a solid electrolyte which does not contain theswelling solvent is employed.

The solid electrolyte must have an ion conductivity of 1 mS/cm orgreater at room temperature. When the solid electrolyte has theforegoing characteristic, its chemical structure is not limited. Thesolid electrolyte of the foregoing type is exemplified by an organicsolid electrolyte obtained by dissolving an inorganic salt in any one ofpolyvinylidene fluoride, polyacrylonitrile, polyethylene oxide, apolysiloxane compound, a polyphosphagen compound, polypropylene oxide,polymethylmethacrylate, polymethacrylonitrile or a polyether compound;an ion-conductive ceramic material or ion-conductive glass.

The shape of the gel electrolyte battery 1 according to this embodimentis not limited. For example, a cylindrical shape, a rectangular shape, acoin shape or the like may be employed. The present invention may beapplied to both a primary battery and a secondary battery.

Second Embodiment

An example of the structure of a gel electrolyte battery 20 according tothis embodiment is shown in FIGS. 7 and 8. The gel electrolyte battery20 incorporates an elongated positive electrode 21; an elongatednegative electrode 22 disposed opposite to the positive electrode 21; agel electrolyte layer 23 formed on each of the positive electrode 21 andthe negative electrode 22; and a separator 24 disposed between thepositive electrode 21 having the gel electrolyte layer 23 formed thereonand the negative electrode 22 having the gel electrolyte layer 23 formedthereon.

The gel electrolyte battery 20 incorporates the positive electrode 21having the gel electrolyte layer 23 formed thereon and the negativeelectrode 22 having the gel electrolyte layer 23 formed thereon. Thepositive electrode 21 and the negative electrode 22 are laminated by theseparator 24 and wound in the lengthwise direction so that a woundelectrode 25 having the structure shown in FIG. 9 is formed. The woundelectrode 25 is covered with a casing film 26 made of an insulatingmaterial, so that the wound electrode 25 is sealed by the casing film26. A positive-electrode terminal 27 is connected to the positiveelectrode 21, while a negative-electrode terminal 28 is connected to thenegative electrode 22. The positive-electrode terminal 27 and thenegative-electrode terminal 28 are sandwiched in a sealing opening whichis the outer periphery of the casing film 26. The regions in which thepositive-electrode terminal 27 and the negative-electrode terminal 28are in contact with the casing film 26 are provided with resin film 29.

As shown in FIG. 10, the positive electrode 21 incorporates apositive-electrode active material layer 21 a containing apositive-electrode active material and formed on a positive electrodecollector 21 b. The positive electrode collector 21 b may be constitutedby a metal foil, such as an aluminum foil.

The positive-electrode active material may be a material which permitsimplantation and separation of positive ions. Ions above are exemplifiedby Li ions. Specifically, the positive-electrode active material isexemplified by LiCoO₂, LiNiO₂ and LiMn₂O₄. Two or more types oftransition metal elements may be employed as well as use of a soletransition metal element. Specifically, LiNi_(0.5)Co_(0.5)O₂ or the likemay be employed.

The positive-electrode active material layer 21 a is formed as follows:the foregoing positive-electrode active material, a carbon materialserving as a conductive material and polyvinylidene fluoride serving asa binder are mixed. Then, N-methylpyrolidone serving as solvent is addedso that paste is prepared. The obtained paste is uniformly applied tothe surface of the aluminum foil which is formed into the positiveelectrode collector by a doctor blade method. Then, the aluminum foil isdried at high temperatures so that the N-methylpyrolidone is removed.

As for the mixture ratio of the positive-electrode active material, theconductor, the binder and N-methylpyrolidone, the mixture ratio is notlimited. The necessity lies in that a paste is realized in which themixture is uniformly dispersed.

FIG. 10 shows a state where a gel electrolyte layer 23 to be describedlater is formed on the positive-electrode active material layer 21 a ofthe positive electrode 21.

As shown in FIG. 11, the negative electrode 22 incorporates anegative-electrode active material layer 22 a containing anegative-electrode active material and formed on the negative electrodecollector 22 b. The negative electrode collector 22 b is constituted by,for example, a metal foil, such as a copper foil.

The negative-electrode active material may be a material which permitsimplantation and separation of Li and which is exemplified by graphite,non-graphitizable carbon or graphitizable carbon.

The negative-electrode active material layer 22 a is formed as follows:the foregoing negative-electrode active material and polyvinylidenefluoride serving as the binder are mixed with each other. Then,N-methylpyrolidone serving as solvent is added to prepare a paste. Theobtained paste is uniformly applied to the surface of a copper foilwhich is formed into a negative-electrode collector by the doctor blademethod. Then, the copper foil is dried at high temperatures to removeN-methylpyrolidone. Thus, the negative-electrode active material layer22 a is formed.

The mixture ratio of the negative-electrode active material, the binderand N-methylpyrolidone is determined to prepare a paste in which themixture is uniformly dispersed. Therefore, the mixture ratio is notlimited. “Permitting insertion and separation of Li” is not limited toinsertion and removal of Li with respect to the inside portion of thecrystal structure. When charge and discharge are permitted in a case ofa completed battery, a determination is made that implantation andseparation can be performed. The negative electrode is exemplified by aLi negative electrode and a Li—Al alloy negative electrode.

FIG. 11 shows a state where a gel electrolyte layer 23 to be describedlater has been formed on the negative-electrode active material layer 22a of the negative electrode 22.

The gel electrolyte layer 23 contains an electrolyte salt, a matrixpolymer and a solvent serving as a plasticizer.

The matrix polymer must have compatibility with the solvent. Thematerial is exemplified by polyacrylonitrile, a polyethylene oxidepolymer, polyvinylidene fluoride, a copolymer of polyvinylidene fluorideand hexafluoropropylene, styrene-butadiene rubber andpolymethylmethacrylate. Two or more types of matrix polymers may beemployed as well as the use only one type. A polymer which is notincluded in the foregoing examples, which has compatibility with thesolvent and which is formed into gel may be employed.

The solvent is a solvent which can be dispersed in the matrix polymer. Anonaqueous solvent is exemplified by ethylene carbonate, propylenecarbonate, butylene carbonate, γ-butylolactone, diethyl carbonate,dimethyl carbonate, ethylmethyl carbonate and dimethoxyethane. Only onetype of the foregoing materials may be employed as the solvent or two ormore types of the foregoing materials may be employed.

The electrolyte salt must be soluble in the foregoing solvent. Suitablecations are exemplified by alkali metal and alkaline earth metalcations. Suitable anions are exemplified by Cl⁻, Br⁻, I⁻, SCN⁻, ClO₄ ⁻,BF₄ ⁻, PF₆ ⁻, CF₃SO₃ ⁻ and (CF₃SO₂)₂N⁻ anions. The concentration of theelectrolyte salt must be a concentration such that the electrolyte saltcan be dissolved in the solvent.

It is preferable that the thickness of the gel electrolyte layer 23 isfrom 5 μm or greater to 15 μm or smaller. When the thickness of the gelelectrolyte layer 23 is less than 5 μm, short circuiting caused bycontact between the positive electrode and the negative electrode cannotbe prevented. When the thickness of the gel electrolyte layer 23 isgreater than 15 μm, resistance against a large load deteriorates and thevolume energy density is lessened.

The separator 24 is disposed between the positive electrode 2 and thenegative electrode 3 to prevent short circuiting caused by the physicalcontact between the positive electrode 2 and the negative electrode 3.The separator 24 according to this embodiment is constituted by acomposite material of polyethylene and polypropylene obtained by addingpolypropylene to polyethylene. When the separator 24 is constituted bythe composite material of polyethylene and polypropylene, the melt-downtemperature can be raised, while maintaining the shutdown temperature ofthe separator 24 similar to that of a polyethylene separator.

Specifically, it is preferable that the melt-down temperature of theseparator 24 is greater than the melt-down temperature of thepolyethylene separator by from 10° C. or greater to 30° C. or less. Themelt-down temperature is a temperature at which the shutdown separator24 is melted and broken.

When the melt-down temperature of the separator 24 is greater than themelt-down temperature of the polyethylene separator by less than 10° C.,the effect of the present invention to raise the melt-down temperaturecannot satisfactorily be obtained. The reason why the melt-downtemperature of the separator 24 is greater than the melt-downtemperature of the polyethylene separator by not greater than 30° C.will now be described. That is, the difference between the melt-downtemperature of the polypropylene separator and that of the polyethyleneseparator is about 30° C.

It is preferable that the thickness of the separator 24 according tothis embodiment is from 5 μm or greater to 15 μm or less. When thethickness of the separator 24 is less than 5 μm, the separator 24 cannoteasily be handled when the battery is assembled. Thus, the manufacturingyield of the battery deteriorates. When the thickness of the separator24 is greater than 15 μm, the internal resistance of the battery israised. What is worse, the energy density loss is increased undesirably.Therefore, the thickness of the separator 24 is made to satisfy therange from 5 μm or greater to 15 μm or less. Thus, deterioration in themanufacturing yield of the battery, rising of the internal resistance ofthe battery and the energy density loss can be prevented.

It is preferable that the volume porosity of the separator 24 is notless than 25% nor greater than 60%. When the volume porosity of theseparator 24 is less than 25%, the internal resistance of the battery isso great that it prevents obtaining the required output characteristics.When the volume porosity of the separator 24 is greater than 60%,satisfactory mechanical strength of the separator cannot be obtained.Therefore, the volume porosity of the separator 24 is made to satisfythe range not less than 25% nor greater than 60%. Thus, the mechanicalstrength of the separator 24 can be maintained without any increase inthe internal resistance of the battery.

As described above, the separator 24 according to this embodiment isprepared, for example, as follows. Note that the method for preparingthe separator 24 is not limited to the following specific values. Alsothe mixture ratio of the polyethylene and polypropylene which constitutethe separator 24 is not limited to the following value.

Initially, 0.375 part by weight of an oxidation inhibitor is added to100 parts by weight of a polyolefin mixture composed of 20 wt % ofultra-high molecular weight polyethylene (UHMWPE) having a weightaverage molecular weight (Mw) of 2.5×10⁶, 30 wt % of high-densitypolyethylene (HDPE) having a weight average molecular weight (Mw) of3.5×10⁵, and 50 wt % of polypropylene having a weight average molecularweight (Mw) of 5.1×10⁵ so that a polyolefin composition is prepared.

Then, 30 parts by weight of the polyolefin composition is introducedinto a biaxial extruder (having a diameter of 58 mm, L/D=42 and strongkneading type). Moreover, 70 parts by weight of liquid paraffin issupplied through a side feeder so as to be melted and kneaded at 200 rpmso that polyolefin solution is prepared in the extruder.

Then, the polyolefin solution is extruded from a T-die disposed at theleading end of the extruder at 190° C. so as to be wound around acooling roll. Thus, a gel sheet is molded. Then, the gel sheet issimultaneously double-axis oriented at 115° C. to obtain a 5×5 orientedfilm. The obtained oriented film is cleaned with methylene chloride toextract and remove liquid paraffin. Then, the oriented film is dried andsubjected to heat treatment. Thus, a fine-porous separator 24constituted by a composite material of polyethylene and polypropylene isobtained.

The separator 24 according to this embodiment constituted by thecomposite material of polyethylene and polypropylene may be structuredby bonding each of a separator 24 a constituted by polyethylene and aseparator 24 b constituted by polypropylene to each other as shown inFIG. 12. When the separator 24 a constituted by polyethylene and theseparator 24 b constituted by polypropylene are bonded to each other,the melt-down temperature of the separator 24 can be raised to themelting point of the polypropylene while maintaining the shutdowntemperature of the separator 24 which is the temperature ofpolyethylene.

The reason why each of the separator 24 a constituted by polyethyleneand the separator 24 b constituted by polypropylene are bonded to eachother will now be described. When three or more separators are overlaid,the thickness of the separator is increased excessively to preventincrease in the internal resistance of the battery. Thus, there arises aproblem in that the energy density loss is increased undesirably.Therefore, each of the separator 24 a constituted by polyethylene andthe separator 24 b constituted by polypropylene are bonded to each otherso that the thickness of the separator is reduced as much as possible.Thus, a maximum effect can be obtained.

When the separator 24 is formed into the foregoing bonded structure, itis preferable that the thickness of each of the separator 24 aconstituted by polyethylene and the separator 24 b constituted bypolypropylene is not less than 2.5 μm nor greater than 7.5 μm. Moreover,it is preferable that the total thickness of the two separators is notless than 5 μm nor greater than 15 μm. When the thickness of theseparator 24 is less than 5 μm, the separator 24 cannot easily behandled when the battery is assembled. Thus, the manufacturing yield ofthe battery deteriorates. When the thickness of the separator 24 isgreater than 15 μm, the internal resistance of the battery is raisedexcessively. What is worse, there arises a problem in that the energydensity loss is increased. Therefore, the thickness of the separator 24is made to satisfy the range not less than 5 μm nor greater than 15 μm.Thus, deterioration in the manufacturing yield of the battery, increasein the internal resistance of the battery and great energy density losscan be prevented.

The separator 24 according to this embodiment must have a width greaterthan the width of each of the positive electrode 22 and the negativeelectrode 23, as shown in FIG. 13. When the positive electrode 22, thenegative electrode 23 and the separator 24 are overlaid and wound,deviations of the positive electrode 22, the negative electrode 23 andthe separator 24 sometimes occur. Assuming that the amount of thewidthwise directional deviations are L₁ and L₂ when the positiveelectrode 22, the negative electrode 23 and the separator 24 have beenoverlaid as shown in FIG. 13, the positive electrode 22 and the negativeelectrode 23 are brought into contact with each other when L₁<0 or L₂<0.That is, when the end of the separator 24 is shorter than the end of thepositive electrode 22 or the negative electrode 23, contact occurs. As aresult, internal short circuiting occurs, causing the manufacturingyield of the battery to deteriorate.

Therefore, when deviation takes place in a process for overlaying andwinding the positive electrode 22, the negative electrode 23 and theseparator 24, occurrence of the contact between the positive electrode22 and the gel electrolyte layer 23 must be prevented. Thus, the widthof the separator 24 must be somewhat greater than the width of each ofthe positive electrode 22 and the negative electrode 23. In a case wherethe width of the separator 24 is excessively increased, the energydensity of the battery is lessened. Therefore, the width of theseparator 24 must be determined in such a manner that L₁>0.5 mm, L₂>0.5mm and L₁+L₂<4 mm as shown in FIG. 13. When the separator 24 has theforegoing width, occurrence of internal short circuiting caused by thepositive electrode 22 and the gel electrolyte layer 23 can be preventedin case of deviation between the positive electrode 22, the negativeelectrode 23 and the separator 24. As a result, deterioration in themanufacturing yield can be prevented.

The gel electrolyte battery 20 incorporating the above-mentionedseparator 24 and according to this embodiment is prepared as follows.

Initially, the positive electrode 21 is prepared as follows: apositive-electrode mix containing a positive-electrode active materialand a binder is uniformly applied to the surface of a metal foil, suchas an aluminum foil, which is formed into the positive electrodecollector 21 b. Then, the metal foil is dried so that thepositive-electrode active material layer 21 a is formed. Thus, a sheetof the positive electrode 2 is prepared. The binder of thepositive-electrode mix may be a conventional binder. Note that aconventional additive may be added to the positive-electrode mix.

Then, the gel electrolyte layer 23 is formed on the positive-electrodeactive material layer 21 a of the positive electrode sheet. To form thegel electrolyte layer 23, an electrolyte salt is dissolved in anonaqueous solvent to prepare a nonaqueous electrolytic solution. Thematrix polymer is added to the nonaqueous electrolytic solution, andthen the solution is sufficiently stirred so as to dissolve the matrixpolymer. Thus, a sol electrolyte solution is prepared.

Then, the electrolyte solution is applied to the surface of thepositive-electrode active material layer 21 a in a predeterminedquantity. Then, the positive-electrode active material layer 21 a iscooled at room temperature to gel the matrix polymer. Thus, the gelelectrolyte layer 23 is formed on the positive-electrode active materiallayer 21 a.

Then, the positive electrode sheet having the gel electrolyte layer 23formed thereon is cut to obtain an elongated member. Then, a lead wireconstituted by, for example, aluminum, is welded to a portion of thepositive electrode collector 21 b in which the positive-electrode activematerial layer 21 a is not formed so that the positive-electrodeterminal 27 is formed. Thus, an elongated positive electrode 21 havingthe gel electrolyte layer 23 formed thereon can be obtained.

The negative electrode 22 is prepared as follows: a negative-electrodemix containing a negative-electrode active material and a binder isuniformly applied to a metal foil, such as a copper foil, which isformed into the negative electrode collector 22 b. Then, the metal foilis dried so that the negative-electrode active material layer 22 a isformed. Thus, a negative electrode sheet is prepared. The binder of thenegative-electrode mix may be a conventional binder. Note that aconventional additive may be added to the negative-electrode mix.

Then, the gel electrolyte layer 4 is formed on the negative electrodecollector 22 b of the negative electrode sheet. To form the gelelectrolyte layer 23, an electrolyte solution prepared similarly to theforegoing process is applied to the negative-electrode active materiallayer in a predetermined quantity. Then, the negative-electrode activematerial layer is cooled at room temperature so as to gel the matrixpolymer. As a result, the gel electrolyte layer 23 is formed on thenegative electrode collector 22 b.

Then, the negative electrode sheet having the gel electrolyte layer 4formed thereon is cut to obtain an elongated member. Then, a lead wireconstituted by, for example, nickel, is welded to a portion of thenegative electrode collector 22 b in which the negative-electrode activematerial layer 22 a is not formed so that the negative-electrodeterminal 28 is formed. Thus, the elongated negative electrode 3 havingthe gel electrolyte layer 23 formed thereon can be obtained.

The surfaces of the thus-prepared elongated positive electrode 21 andthe negative electrode 22 on each of which the gel electrolyte layer 23are disposed opposite to each other. Then, the separator 24 is disposedbetween the positive electrode 21 and the negative electrode 22 so as tobe pressed. Thus, an electrode laminate is formed. Then, the electrodelaminate is wound in the lengthwise direction so that the woundelectrode 25 is formed.

Finally, the wound electrode 25 is sandwiched by the casing film 26constituted by an insulating material. Then, the resin film 29 isdisposed in each of the regions in which the positive-electrode terminal27 and the negative-electrode terminal 28 overlap the casing film 26.Then, the outer periphery of the casing film 26 is sealed to insert thepositive-electrode terminal 27 and the negative-electrode terminal 28into the sealing portion of the casing film 26. Moreover, the woundelectrode 25 is hermetically enclosed in the casing film 26. Thus, thegel electrolyte battery 20 is prepared.

The casing film 26 is formed by sequentially laminating a firstpolyethylene terephthalate layer 26 a, an aluminum layer 26 b, a secondpolyethylene terephthalate layer 26 c and a straight-chain andlow-density polyethylene layer 26 d in this order, as shown in FIG. 14.The straight-chain and low-density polyethylene layer 26 d serves as athermal bonding layer. When the wound electrode 25 is enclosed, thestraight-chain and low-density polyethylene layer 26 d is disposed onthe inside. The thermal bonding layer may be constituted by a material,such as polyethylene terephthalate, nylon, cast polypropylene orhigh-density polyethylene as well as straight-chain and low-densitypolyethylene.

Note that the structure of the casing film 26 is not limited to theforegoing structure. The necessity is such that at least one aluminumlayer is present in the layer and the thermo-bonding polymer film ispresent on at least one surface.

When the wound electrode 25 is packed in the casing film 26, the resinfilm 29 is disposed in each of the regions in which the casing film 26and the positive-electrode terminal 27 and the negative-electrodeterminal 28 are brought into contact with each other. Thus, occurrenceof short circuiting caused by burrs of the casing film 26 can beprevented. Moreover, adhesion between the casing film 26 and thepositive-electrode terminal 27 and between the casing film 26 and thenegative-electrode terminal 28 can be improved.

The material of the resin film 29 must adhere to the positive electrodeterminal 17 and the negative electrode terminal 18. When the foregoingrequirement is satisfied, the material is not limited. It is preferablethat any one of the following materials is employed: polyethylene,polypropylene, denatured polyethylene, denatured polypropylene, theircopolymers and polyolefin resin. It is preferable that the thickness ofthe resin film 29 is from 20 μm to 300 μm realized before the thermalbonding operation is performed. When the thickness of the resin film 29is less than 20 μm, ease of handling deteriorates. When the thickness isgreater than 300 μm, water penetration easily takes place. Thus, theairtightness in the battery cannot easily be maintained.

In the foregoing embodiment, the elongated positive electrode 21 and theelongated negative electrode 22 are laminated. Then, the laminate iswound in the lengthwise direction so that the wound electrode 25 isformed. The present invention is not limited to the foregoing structure.A rectangular positive electrode 21 and a rectangular negative electrode22 may be laminated to form a wound electrode. Another structure may beemployed in which the electrode laminate is alternately folded.

In the foregoing embodiment, the electrolyte to be interposed betweenthe positive electrode 21 and the negative electrode 22 is the gelelectrolyte containing the matrix polymer, the electrolyte salt and thesolvent. The present invention is not limited to the foregoingstructure. The present invention may be applied to a structure in whicha solid electrolyte which does not contain solvent is employed and astructure in which an electrolytic solution which does not contain thematrix polymer is employed.

The shape of the gel electrolyte battery 20 according to this embodimentis not limited. For example, a cylindrical shape, a rectangular shape ora coin shape may be employed. Moreover, the size may be a variety ofsizes including a thin structure and a large-size structure. The presentinvention may be applied to both primary batteries and secondarybatteries.

EXAMPLES

To confirm the effects of the invention, batteries having the foregoingstructures were prepared to evaluate their characteristics.

First Experiment

In Examples 1 to 7 and Comparative Example 1, the separators accordingto the first embodiment were used to prepare batteries so as to evaluatetheir characteristics.

Manufacturing of Sample Batteries

Example 1

Initially, the positive electrode was prepared.

Initially, commercially available lithium carbonate and cobalt carbonatewere mixed with each other in such a manner that the composition ratioof lithium atoms and cobalt atoms was 1:1. Then, the mixture was bakedfor five hours in air at 900° C. Thus, cobalt acid lithium serving asthe positive-electrode active material was obtained. The mean particlesize of the cobalt acid lithium was 10 μm.

Then, 91 parts by weight of the obtained positive-electrode activematerial, 6 parts by weight of graphite serving as a conductive materialand 3 parts by weight of polyvinylidene fluoride serving as the binderwere mixed with one another so that a positive-electrode mix wasprepared. Then, the positive-electrode mix was dispersed inN-methyl-2-pyrolidone so as to be formed into a paste.

Then, the obtained positive-electrode mix paste was uniformly applied tothe two sides of an elongated aluminum foil serving as the positiveelectrode collector and having a thickness of 20 μm. Then, the aluminumfoil was subjected to a drying process. After the drying process wascompleted, a roller press was operated to compress and mold the aluminumfoil. Thus, a positive-electrode active material layer having athickness of 40 μm was formed. Then, a lead wire constituted by aluminumwas welded to a portion of the positive electrode collector in which thepositive-electrode active material layer was not formed. Thus, apositive electrode terminal was formed. As a result, the positiveelectrode was prepared. The density of the positive-electrode activematerial layer was 3.6 g/cm³.

Then, the negative electrode was prepared as follows.

Initially, methocarbon microbeads having a mean particle size of 25 μmwere baked at 2800° C. so that graphite serving as thenegative-electrode active material was obtained.

Then, 90 parts by weight of the obtained negative-electrode activematerial and 10 parts by weight of polyvinylidene fluoride were mixed sothat a negative-electrode mix was prepared. Then, the negative-electrodemix was dispersed in N-methyl-2-pyrolidone serving as the solvent so asto be formed into a paste.

Then, the obtained negative-electrode mix paste was uniformly applied tothe two sides of an elongated copper foil serving as the negativeelectrode collector and having a thickness of 15 μm. Then, a dryingprocess was performed. After the elongated copper foil had been dried, aroller press was operated to perform a compression molding operation.Thus, a negative-electrode active material layer having a thickness of55 μm was formed. Then, a nickel lead wire was welded to a portion ofthe negative electrode collector in which the negative-electrode activematerial layer was not formed so that a positive electrode terminal wasformed. The density of the negative-electrode active material layer was1.6 g/cm³ at this time.

Then, a gel electrolyte layer was formed on each of the positiveelectrode and the negative electrode.

Initially, 80 g of dimethyl carbonate, 40 g of ethylene carbonate, 40 gof propylene carbonate, 9.2 g of LIPF₆, 0.8 g of vinylene carbonate and0.8 g of 2,4-difluoroanisol were mixed with each other so that asolution was prepared. Then, the solution was added to 10 g of acopolymer (copolymerization weight ratio PVdF:HFP=97:3) ofpolyvinylidene fluoride (PVdF) and hexafluoropolypropylene (HFP). Then,a homogenizer was used to prepare a uniform dispersion. Then, heatingand stirring were performed at 75° C. until a colorless and transparentstate was realized. Thus, the electrolyte solution was prepared.

Then, the obtained electrolyte solution was uniformly applied to the twosides of each of the positive electrode and the negative electrode bythe doctor blade method. Then, the positive electrode and the negativeelectrode applied with the electrolyte solution were allowed to stand ina drying unit, the inside portion of which was maintained at 40° C., forone minute. Thus, the electrolyte solution was gelled so that a gelelectrolyte layer having a thickness of about 8 μm was formed on each ofthe two sides of each of the positive electrode and the negativeelectrode.

Then, the battery was assembled as follows.

Initially, the thus-prepared elongated positive electrode incorporatingthe gel electrolyte layer formed on each of the two sides thereof andthe elongated negative electrode incorporating the gel electrolyte layerformed on each of the two sides thereof were laminated through aseparator so that a laminate was obtained. Then, the laminate was woundin its lengthwise direction so that a wound electrode was obtained. Theseparator was a porous polyethylene film having a volume porosity of 36%and a thickness of 8 μm.

The wound electrode was sandwiched by a moisture-proof casing filmformed by laminating a nylon sheet having a thickness of 25 μm, analuminum sheet having a thickness of 40 μm and a polypropylene sheethaving a thickness of 30 μm. Then, the outer periphery of the casingfilm was bonded with heat under reduced pressure so as to be sealed.Thus, the wound electrode was hermetically enclosed in the casing film.At this time, the positive electrode terminal and the negative electrodeterminal were sandwiched in the sealing regions of the casing film.Moreover, a polyolefin film was disposed in each of the regions in whichthe casing film and the positive electrode terminal and the negativeelectrode terminal were in contact with each other.

Finally, the electrode elements were subjected to heat treatment in astate where the electrode terminals were enclosed in the casing film.Thus, the gel electrolyte battery was prepared.

Example 2

A process similar to that according to Example 1 was performed exceptfor the separator which was, in this example, a porous polyethylene filmhaving a volume porosity of 37% and a thickness of 9 μm. Thus, a gelelectrolyte battery was prepared.

Example 3

A process similar to that according to Example 1 was performed exceptfor the separator which was, in this example, a porous polyethylene filmhaving a volume porosity of 35% and a thickness of 10 μm. Thus, a gelelectrolyte battery was prepared.

Example 4

A process similar to that according to Example 1 was performed exceptfor the separator which was, in this example, a porous polyethylene filmhaving a volume porosity of 30% and a thickness of 12 μm. Thus, a gelelectrolyte battery was prepared.

Example 5

A process similar to that according to Example 1 was performed exceptfor the separator which was, in this example, a porous polyethylene filmhaving a volume porosity of 39% and a thickness of 15 μm. Thus, a gelelectrolyte battery was prepared.

Example 6

A process similar to that according to Example 1 was performed exceptfor the separator which was, in this example, a porous polyethylene filmhaving a volume porosity of 36% and a thickness of 8 μm. Thus, a gelelectrolyte battery was prepared.

Example 7

A process similar to that according to Example 1 was performed exceptfor the separator which was, in this example, a porous polyethylene filmhaving a volume porosity of 36% and a thickness of 16 μm. Thus, a gelelectrolyte battery was prepared.

Comparative Example 1

A process similar to that according to Example 1 was performed exceptfor omission of the separator in this comparative example. Thus, a gelelectrolyte battery was prepared.

Evaluation of Characteristics of Sample Batteries

The materials, volume porosities, thicknesses, breaking strength andbreaking ductility of the separators according to Examples 1 to 7 arecollectively shown in Table 1.

TABLE 1 Vacancy Breaking Breaking Ratio Thick- Strength DuctilityMaterial (%) ness (kg/cm²) (%) Example 1 polyethylene 36 8 739 161Example 2 polyethylene 37 9 1185 156 Example 3 polyethylene 35 10 1300164 Example 4 polyethylene 30 12 1409 170 Example 5 polyethylene 39 151179 137 Example 6 polypropylene 36 8 1650 139 Example 7 polypropylene36 16 1946 127

Evaluation of Charge and Discharge Characteristics

The thus-prepared batteries were subjected to charge and discharge testsso that the characteristics of the batteries were evaluated. Apotentio-galvanostat was operated to perform the charge and dischargetests of the batteries. A constant-current and constant-voltage methodwas employed to perform the charge and discharge.

Initially, each battery was charged with a constant current of 200 mA.When the voltage of the closed circuit was raised to 4.2 V, theconstant-current charge was changed to the constant-voltage charge.Then, the constant-voltage charge was continued. The charge wascompleted nine hours after start of the charging operation. Then,discharge was performed with a constant current of 200 mA. When thevoltage of the closed circuit was raised to 3.0 V, the discharge wascompleted.

The charge and discharge capacities of each battery were detected.Moreover, a charge and discharge efficiency and an energy density ofeach battery were calculated.

The detected charge capacity, the discharge capacity, the charge anddischarge efficiency and the energy density of each of the batteriesaccording to Examples 1 to 7 and Comparative Example 1 are shown inTable 2.

TABLE 2 Initial Charge and Initial Charge Discharge Discharge CapacityCapacity Efficiency Energy Density (mAh/g) (mAh/g) (%) (Wh/l) Example 1710 611 86 332 Example 2 709 608 86 331 Example 3 711 606 85 330 Example4 708 609 86 331 Example 5 710 606 85 330 Example 6 497 352 71 189Example 7 512 355 69 191 Comparative 1503 349 23 187 Example 1

As can be understood from Table 2, the batteries according to Examples 1to 7 were excellent in all of the charge capacity, the dischargecapacity, the charge and discharge efficiency and the energy density.Thus, the desired excellent characteristics were realized. Inparticular, the batteries each incorporating the polyethylene separatorand according to Examples 1 to 5 were excellent in the characteristics.

On the other hand, the battery according to Comparative Example 1encountered small short circuit during the charging operation. That is,satisfactory battery characteristics were not obtained.

Evaluation of Safety of Sample Batteries

The shutdown start temperature of the separator of each of the batteriesaccording to Examples 1 to 7 and Comparative Example 1 and the highestsurface temperature of each battery when the battery was externallyshort-circuited were examined.

The shutdown temperature was measured such that the battery was heatedat a rising ratio of 5° C./minute. When the AC resistance was raised by10° C. or more owing to application of 1 kHz, the temperature of eachbattery was measured.

The temperature of the surface of the battery realized when the batterywas externally short-circuited was measured such that the battery wascharged under similar conditions to those for the charge and dischargetests. Then, the battery was heated to 60° C. In the foregoing state,the highest temperature of the battery realized when the terminals wereshort-circuited by using a 12 mΩ resistor was measured by using athermo-couple.

The shutdown temperature and the temperature of the surface of each ofthe batteries according to Examples 1 to 7 and Comparative Example 1realized when external short circuiting was caused to occur are shown inTable 3.

TABLE 3 Temperature of Surface of Battery when Shutdown Start ExternalShort Temperature Circuit was Caused (° C.) to Occur (° C.) Example 1126 118 Example 2 126 119 Example 3 125 117 Example 4 126 118 Example 5123 116 Example 6 163 161 Example 7 165 165 Comparative — 200 or higherExample 1

As can be understood from Table 3, each of the batteries according toExamples 1 to 5 and incorporating the separator constituted bypolyethylene encountered shutdown in a temperature range from 100° C. to160° C. The relationship between the temperature of the batteryaccording to Example 1 and the impedance in the battery was shown inFIG. 15. As can be understood from FIG. 15, the impedance in the batterywas rapidly increased when the temperature of the battery was about 126°C.

The batteries according to Examples 6 and 7 and each incorporating theseparator constituted by polypropylene encountered shutdown in spite ofthe temperature being 160° C. or greater. The relationship between thetemperature of the battery according to Example 6 and the impedance inthe battery was shown in FIG. 16. As can be understood from FIG. 16, theimpedance in the battery was rapidly increased when the temperature ofthe battery was about 163° C.

The temperature of the surface of each of the batteries according toExamples 1 to 5 when the charged battery was externally short-circuitedwas 120° C. or less. Thus, heat generation occurring when the battery iserroneously operated was effectively prevented. Hence it follows thatthe safety of the battery was secured. On the other hand, thetemperature of the surface of each of the batteries according toExamples 6 and 7 was raised to 160° C. when the charged battery wasexternally short-circuited. Thus, great heat generation occurs when thebattery is used erroneously. The batteries according to Examples 1 to 7encountered the shutdown effect were free from any smoke from the insideportion of the battery when the charged battery was externallyshort-circuited.

On the other hand, the battery according to Comparative Example 1 wasfree from any shutdown effect when the temperature of the battery wasraised to 180° C. When the charged battery was externallyshort-circuited, the temperature of the surface of the battery wasraised to 200° C. Moreover, smoke from the inside portion of the batteryoccurred.

Specification of Physical Properties of Separator

The relationship between the breaking strength and the breakingductility of the separator of each of the batteries according toExamples 1 to 7 is shown in FIG. 17. As can be understood from FIG. 17and the results of the evaluation of the characteristics of the battery,the breaking strength of the separator of each of the batteriesaccording to Examples 1 to 5 and from which excellent characteristics ofthe battery were obtained was less than 1650 kg/cm². Moreover, thebreaking ductility was 135% or greater. The foregoing mechanicalstrength was realized.

All of the separators each having the foregoing mechanical strength hadthe fibril structure shown in FIG. 6. An electronic microscopephotograph of the fine structure of the separator according to Example 6at a magnification of 50,000 times is shown in FIG. 18. When acomparison between FIGS. 6 and 18 was made, it can be understood thatthe mechanical strength of the separator relates to its fine structure.To realize the foregoing mechanical strength, the fine structure of theseparator must be the fibril structure.

As a result, use of the porous polyolefin separator having a thicknessnot less than 5 μm nor greater than 15 μm, a volume porosity not lessthan 25% nor greater than 60% and a shutdown effect when the temperatureof the battery is from 100° C. or greater to 160° C. or less enabledboth of a high energy density and safety of the battery to be realized.

It can also be understood that use of the porous polyolefin film havinga thickness not less than 5 μm nor greater than 15 μm, a volume porositynot less than 25% nor greater than 60%, a breaking strength less than1650 kg/cm² and a breaking ductility not less than 135% enabled both ahigh energy density and safety of the battery to be realized.

Second Experiment

In each of the following Examples 8 and 9 and Comparative Example 2, theseparator according to the second embodiment was employed to preparebatteries to evaluate their characteristics.

Example 8

The positive electrode was prepared such that 95 wt % of LiCoO₂ servingas the positive-electrode active material, 2 wt % of graphite serving asa conductive material and 3 wt % of polyvinylidene fluoride were mixedwith one another. Thus, a positive-electrode mix was prepared. Then,N-methyl pyrolidone was added in a quantity which was 0.6 times thequantity of the positive-electrode mix so that a paste was prepared.

Then, the obtained paste was uniformly applied to either side of thealuminum foil which was formed into the positive electrode collector bythe doctor blade method. Then, the aluminum foil was dried at hightemperatures to remove N-methyl pyrolidone. Thus, a positive-electrodeactive material layer was formed. Finally, a roll press was operated toapply sufficient polyethylene to perform a pressing operation. Then, thesample was cut to a size 300 mm×50 mm so that the positive electrode wasprepared. An aluminum wire was spot-welded to the positive electrode sothat a positive electrode terminal was formed.

To prepare the negative electrode, 91 wt % of graphite serving as thenegative-electrode active material and 9 wt % of polyvinylidene fluorideserving as the binder were mixed with each other so that thenegative-electrode mix was prepared. Then, N-methyl pyrolidone was addedin a quantity which was 1.1 times the quantity of the negative-electrodemix so that a paste was prepared.

Then, the obtained paste was uniformly applied to either side of thecopper foil which was formed into the negative electrode collector bythe doctor blade method. Then, the copper foil was dried to removeN-methyl pyrolidone so that the negative-electrode active material layerwas formed. Finally, the roll press was operated to apply sufficientpolyethylene to perform a pressing operation. Then, the sample was cutto a size 370 mm×52 mm so that the negative electrode was prepared.Then, a copper wire was spot-welded to the negative electrode so thatthe negative electrode terminal was formed.

On the other hand, 6.7 wt % of polyvinylidene fluoride, 9.2 wt % ofethylene carbonate, 11.6 wt % of propylene carbonate, 2.3 wt % ofγ-butylolactone, 6.67 wt % of dimethyl carbonate and 3.5 wt % of LiPF₆were mixed with one another. Thus, a polymer electrolyte solution wasprepared. Note that dimethyl carbonate served as solvent for dissolvingpolyvinylidene fluoride.

The obtained polymer electrolyte solution was applied to the surface ofeach of the positive electrode and the negative electrode by the doctorblade method. Then, the positive electrode and the negative electrodewere dried for three minutes in a constant-temperature tank set to 35°C. Thus, a thin film was formed. At this time, dimethyl carbonate wasnot left in the polymer electrolyte. The application operation wasperformed in such a manner that the thickness of the polymer electrolyteon each of the positive electrode and the negative electrode was 10 μm.

The separator was a separator constituted by a composite material ofpolyethylene and polypropylene and having a thickness of 10 μm. Theratio of polyethylene and polypropylene in the composite material was1:1. The separator constituted by the composite material was prepared asfollows.

Initially, 100 parts by weight of a polyolefin mixture composed of 20 wt% of ultra high molecular weight polyethylene (UHMWPE) having a weightaverage molecular weight (Mw) of 2.5×10⁶, 30 wt % of high-densitypolyethylene (HDPE) having a weight average molecular weight (Mw) of3.5×10⁵ and 50 wt % of polypropylene having a weight average molecularweight (Mw) of 5.1×10⁵ was added with 0.375 part by weight of aoxidation inhibitor so that a polyolefin composition was prepared.

Then, 30 parts by weight of the polyolefin composition were introducedinto a biaxial the extruder (having a diameter of 58 mm, L/D=42 and astrong kneading type). Moreover, 70 parts by weight of liquid paraffinwere supplied through a side feeder of the biaxial extruder so as to bemelted and kneaded at 200 rpm. Thus, a polyolefin solution was preparedin the extruder.

Then, the polyolefin solution was extruded from a T-die disposed at theleading end of the extruder at 190° C. so as to be wound around acooling roll. Thus, a gel sheet was molded. The, the gel sheet wassimultaneously double-axis oriented at 115° C. to obtain a 5×5 orientedfilm. The obtained oriented film was cleaned with methylene chloride toextract and remove liquid paraffin. Then, the oriented film was driedand subjected to heat treatment. Thus, a fine-porous separatorconstituted by a composite material of polyethylene and polypropylenewas obtained.

The thus-prepared elongated positive electrode having the gelelectrolyte layer and the elongated negative electrode having the gelelectrolyte layer were laminated through the separator so that alaminate was formed. Then, the laminate was wound in its lengthwisedirection. Thus, a 36 mm×52 mm×5 mm wound electrode was obtained.

Then, the wound electrode was sandwiched by a casing film constituted bya moisture-proof multilayered film having a thickness of 100 μm. Then,the outer periphery of the casing film was heat-bonded under reducedpressure so as to be sealed. Thus, the wound electrode was hermeticallysealed in the casing film. At this time, the positive electrode terminaland the negative electrode terminal were sandwiched in the sealingportions of the casing film.

Example 9

A process similar to that according to Example 8 was performed exceptfor a separator which was, in this example, obtained by bonding apolyethylene separator having a thickness of 5 μm and a polypropyleneseparator having a thickness of 5 μm to each other. Thus, a battery wasprepared.

Comparative Example 2

A process similar to that according to Example 8 was performed exceptfor a separator which was, in this example, a polyethylene separatorhaving a thickness of 10 μm. Thus, a battery was prepared.

Each of the thus-prepared batteries were charged and discharged severaltimes. In a discharged state, the battery was introduced into aconstant-temperature tank. While measuring the resistance with 1 kHz,the temperature was raised to 140° C., 140° C., 150° C., 155° C., 160°C., 165° C. and 170° C. at a rising rate of 5° C./minute. Then, eachtemperature was maintained for 30 minutes. When the resistance was notdecreased in the period in which the predetermined temperature wasmaintained, a determination was made that no short circuiting occurred.When the resistance was decreased, a determination was made that shortcircuiting occurred.

Results are shown in Table 4.

TABLE 4 Comparative Example 8 Example 9 Example 2 140° C. ◯ ◯ ◯ 145° C.◯ ◯ ◯ 150° C. ◯ ◯ X 155° C. ◯ ◯ X 160° C. ◯ ◯ X 165° C. ◯ ◯ X 170° C. XX X

The batteries according to Examples 8 and 9 incorporated the separatorconstituted by the composite material of polyethylene and polypropyleneand the separator constituted by bonding the polyethylene separator andthe polypropylene separator to each other. As compared with the batteryaccording to Comparative Example 2 and incorporating the separatorconstituted by only polyethylene, the melt-down temperature was raisedby about 15° C. The battery incorporating the separator having the highmelt-down temperature enabled the temperature at which the internalshort circuiting started owing to the meltdown to be raised. Therefore,when the temperature of the battery was raised, occurrence of theinternal short circuiting can be prevented as compared with thepolyethylene separator. Thus, prevention of heat generation from thebattery caused from the internal short circuiting was permitted.

The prepared battery was charged and discharged several times. Then, thebattery in an overcharged state of 4.4 V was introduced into ahigh-temperature tank. While measuring the resistance with 1 kHz, thetemperature was raised to 135° C., 140° C., 145° C., 150° C. and 155° C.at a rising ratio of 5° C./minute. Each temperature was maintained for30 minutes. When the resistance was not decreased during retention atthe predetermined temperature, a determination was made that no shortcircuiting occurred. When the resistance was decreased, a determinationwas made that short circuiting occurred. Since the voltage was 4.4 V orgreater in the foregoing case, heat was sometimes generated owing toshort circuiting. Therefore, the measurement was completed and it wasthat the resistance was decreased.

Results were shown in Table 5.

TABLE 5 Comparative Example 8 Example 9 Example 2 135° C. ◯ ◯ ◯ 140° C.◯ ◯ X 145° C. ◯ ◯ X 150° C. X ◯ X 155° C. X X X

Even in an abnormal state of overcharging at 4.4 V, the batteriesaccording to Example 8 and Example 9 which incorporated the separatorconstituted by the composite material of polyethylene and polypropyleneand the separator constituted by bonding the polyethylene separator andthe polypropylene separator to each other enabled the melt-downtemperature to be raised by about 15° C. as compared with the batteryaccording to Comparative Example 2. The battery according to ComparativeExample 2 incorporated the separator constituted by only polyethylene.The battery incorporated the separator having the high melt-downtemperature enabled the temperature at which the internal short circuitstarted owing to meltdown to be raised. When the temperature of thebattery was raised, short circuiting did not easily occur as comparedwith the polyethylene separator. Thus, heat generation of the batteryowing to the internal short circuit can be prevented.

In the present invention, the separator constituted by the porouspolyolefin film, the separator constituted by the composite material ofpolyethylene and polypropylene or the separator formed by bonding thefirst separator constituted by polyethylene and the second separatorconstituted by the polypropylene, the mechanical characteristics andthermal characteristics of which are specified are employed. Therefore,both raising of the energy density and improvement in the safety can berealized as distinct from the conventional technique. Thus, ahigh-performance solid electrolyte battery having excellentcharacteristics and safety can be realized.

Although the invention has been described in its preferred form andstructure with a certain degree of particularity, it is understood thatthe present disclosure of the preferred form can be changed in thedetails of construction and in the combination and arrangement of partswithout departing from the spirit and the scope of the invention ashereinafter claimed.

1. A gel electrolyte battery comprising: a positive electrode; anegative electrode disposed opposite to said positive electrode; aseparator disposed between said positive electrode and said negativeelectrode; and gel electrolytes each of which is disposed between saidpositive electrode and said separator and between said separator andsaid negative electrode; wherein said separator comprises a mixture ofpolyethylene and polypropylene, said separator has a thicknesssatisfying a range not less than 5 μm nor greater than 15 μm, theshutdown temperature of said separator is substantially the same as theshutdown temperature of a separator constituted by polyethylene and themeltdown temperature of said separator is greater than the meltdowntemperature of a separator constituted by polyethylene by a rangesatisfying a range not less than 10° C. nor greater than 30° C. whereinthe thickness of the gel electrolytes are not less than 5 μm nor greaterthan 19 μm.
 2. The gel electrolyte battery according to claim 1, whereinsaid gel electrolytes contain a swelling solvent.
 3. The gel electrolytebattery according to claim 1, wherein said electrodes consist of apositive electrode using lithium ions as electrode reaction species anda negative electrode constituted by a carbonaceous material.
 4. The gelelectrolyte battery according to claim 2, wherein said gel electrolytescontain ethylene carbonate, propylene carbonate and LiPF₆.
 5. The gelelectrolyte battery according to claim 4, wherein said gel electrolytescontain vinylene carbonate and/or 2,4-difluoroanisol.
 6. The gelelectrolyte battery according to claim 5, wherein the content of each ofvinylene carbonate and 2,4-difluoroanisol is not greater than 5 wt % ofthe overall weight of said gel electrolytes.
 7. The gel electrolytebattery according to claim 6, wherein Said gel electrolytes are employedwhich is constituted by polyvinylidene fluoride or a copolymer ofpolyvinylidene fluoride.
 8. The gel electrolyte battery according toclaim 7, wherein a copolymer is used which contains polyvinylidenefluoride and hexafluoropolypropylene.
 9. The gel electrolyte batteryaccording to claim 8, wherein said gel electrolytes are composed of acopolymer constituted by polyvinylidene fluoride andhexafluoropolypropylene such that hexafluoropolypropylene is containedin a quantity greater than 8 wt %
 10. The gel electrolyte batteryaccording to claim 7, wherein the separator comprises a polyolefinmixture of 50% wt. polyethylene and 50% wt. polypropylene.
 11. The gelelectrolyte battery according to claim 7, wherein the polyethylenecomprises at least two types of polyethylene of different averagemolecular weight.
 12. The gel electrolyte battery according to claim 11,wherein the polyethylene comprises an ultra-high molecular weightpolyethylene and a high-density polyethylene.
 13. The gel electrolytebattery according to claim 12, wherein the ultra-high molecular weightpolyethylene has an average molecular weight of 2.5×10⁶ and thehigh-density polyethylene has an average molecular weight of 3.5×10⁵.14. The gel electrolyte battery according to claim 10, wherein theseparator comprises a polyolefin mixture of 20% wt. ultra-high molecularweight polyethylene, 30% wt. high-density polyethylene, and 50% wt.polypropylene.